Union muscle helped persuade nearly the entire New York City Assembly delegation to sign on to a letter backing Mayor de Blasio’s tax-the-rich pre-K plan, sources said.

The letter, distributed by Assembly Education Committee Chairwoman Cathy Nolan (D-Queens), may be released at Hizzoner’s pre-K rally today in the state capital.

The plan includes a tax increase for those New York City residents making $500,0000 or more.

Gov. Cuomo, who has proposed his own statewide pre-K expansion and is running for re-election, opposes the tax increase.

The United Federation of Teachers and Health Care Workers Union 1199 “told me they were supporting the mayor’s position and it was important to them,” said one city Democratic Assembly member, who requested anonymity. “The labor leaders were being poignant about their support. It was a very safe position to support the mayor’s legislation.”

Other Assembly members said they signed on to the de Blasio plan on the merits, not because of union pressure.